rust: fix --enable-debug-mutex

--feature is an option for cargo but not for rustc.

Reported-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Bernhard Beschow <shentey@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/meson.build

@@ -7,7 +7,7 @@ if rustc.version().version_compare('>=1.77.0')
   _qemu_api_cfg += ['--cfg', 'has_offset_of']
 endif
 if get_option('debug_mutex')
-  _qemu_api_cfg += ['--feature', 'debug_cell']
+  _qemu_api_cfg += ['--cfg', 'feature="debug_cell"']
 endif
 
 _qemu_api_rs = static_library(